Chronic kidney disease (CKD) has emerged as a silent killer that affects a large segment (15-20%) of the adult population, and is a major risk factor for end-stage renal disease (ESRD), as well as acute kidney injury, cardiovascular disease, and premature death. Progressive tubulointerstitial fibrosis is the final common pathway for all kidney diseases leading to CKD. However, the molecular mechanisms and regulatory steps that govern and modulate tubulointerstitial fibrogenesis are not fully understood. The response to tissue injury involves an ordered sequence of partially overlapping phases: inflammatory, proliferation, and extracellular matrix (ECM) remodeling. Previous work has focused on and demonstrated the key role of macrophages in the response to renal injury, with distinct macrophage subsets regulating the balance of renal injury, inflammation, repair, and fibrosis. Results also have shown a key role for tubular epithelial cells (TECs) in renal fibrogenesis. TECs undergo marked phenotypic changes in acute injury and contribute to both inflammatory and pro-fibrotic phases. Recent and very exciting work suggests a critical role for plasminogen (Plg) and the Plg activation system in macrophage function and macrophage involvement in tissue repair, including promotion of key steps in macrophage phagocytosis and signaling. Results also suggest a direct role for Plg in regulating TEC function, including key TEC phenotypic changes and signaling in response to renal injury. This proposal is based on our proteomics-based discovery of a new protein, the plasminogen receptor, Plg-RKT, which markedly enhances the activation of the zymogen Plg to plasmin, as well as concentrates and localizes the proteolytic activity of plasmin at specific sites on the cell surface. We have observed prominent expression of Plg-RKT in macrophages and TECs. We have developed Plg-RKT-/- mouse models, and in recent studies have demonstrated that Plg-RKT plays a major role in macrophage recruitment and function in response to inflammatory stimulation. In addition, Plg-RKT-/- mice exhibit impaired tissue remodeling and impaired fibrin degradation leading to fibrosis in several in vivo settings. Of note, in recent preliminary studies, we have observed marked increases in renal fibrosis in Plg-RKT-/- mice compared to Plg-RKT+/+ mice in response to acute renal injury. In addition, we have observed that Plg-RKT expression is substantially altered in response to experimental renal injury, and in patients with CKD. The overall objectives of this proposal are to test the hypothesis that Plg-RKT plays a major role in the modulation of renal fibrosis in response to renal injury, and to delineate the specific mechanisms and pathways that mediate the effect of Plg-RKT on renal ECM remodeling and repair. We will use a genetic approach to examine the role of cell-specific Plg-RKT in renal ECM remodeling and the modulation of renal fibrosis in vivo. We will perform studies in Plg-RKT-/- and Plg-RKT+/+ mice, mice in which Plg-RKT is deleted in specific cells, including macrophages and TECs, and mice in which Plg-RKT and fibrinogen are concomitantly deleted, allowing us to examine the role of Plg-RKT at defined steps in the renal injury, remodeling, and fibrosis phases. In addition, we will investigate the role of macrophage and TEC Plg-RKT in key cellular steps that regulate renal ECM remodeling and fibrosis at the cellular level, including regulation of release of pro-inflammatory and anti-inflammatory cytokines from macrophages and TECs, regulation of intracellular signaling pathways, regulation of macrophage phagocytosis, and regulation of key TEC phenotypic changes in response to renal injury. This project seeks to provide new information on Plg- RKT as a pivotal regulator of renal ECM remodeling and repair, with important implications not only for renal fibrosis and CKD, but also a broad array of diseases characterized by dysregulated tissue remodeling and inflammation.